The curious case of the American justice system

The Florida shooting case has left a lot of people seething over the way the law appears to be protecting a guy who brazenly shot an unarmed kid in the name of "self defense" and "standing one's ground". Today I stumbled upon another article in the NYT that talks about a diametrically different aspect of the American justice system - the large number of people incarcerated in US jails. The first line in the article summarizes the gist neatly:

'The United States has less than 5 percent of the world's population. But it has almost a quarter of the world's prisoners.'

I was struck by the absurd irony here. Reading the first story could make you think that hardly anybody - even somebody who brazenly follows an unarmed black kid and shoots him dead - goes to jail. The second story on the other hand paints a completely different picture. What gives?